Chemical vapor deposition (CVD) involves the generation of metal halide gas at low temperatures (e.g. about 100 to 600 degrees C.), introduction of the metal halide gas into a high temperature retort (e.g. 200 to 1200 degrees C. retort temperature), and reaction of the metal halide with substrates positioned in the retort to form a coating thereon. In general, a large excess of metal halide gas is used to prevent reactant starvation in the high temperature coating retort. Metal halide gases are corrosive and/or toxic, and CVD processes are often performed at reduced pressure. Hence, the off gas system (exhaust system) of the coating retort typically includes a liquid ring vacuum pump filled with a caustic solution to neutralize the excess halide gas mixture.
The deposition of excess CVD reactants in the high temperature retort exhaust system can result in rapid attack of the hardware due to resultant liquid metal deposits or blockage of the exhaust system due to solid or sponge metal deposits from condensation and/or decomposition of the reactants. In a high volume CVD coating production operation, such rapid attack and/or blockage of the retort exhaust system involves time consuming and costly cleaning/repair of the exhaust system between coating runs. Historically, two approaches have been used to prevent deposition of excess CVD reactants in the retort exhaust system. The first involves heating the off gas system to prevent deposition. However, since the boiling or sublimation points of many metal halides of interest are high (e.g. greater than 200 degrees C.), this approach is limited by the availability of high temperature corrosion and oxidation resistant hardware, such as tubing, valves, and joints. In addition, the required heating equipment (e.g. heaters, heater controls and the like) increases the cost, complexity and maintenance of the CVD apparatus.
A second approach involves a trap or condenser placed at the beginning of the exhaust system. This approach is common in commercially available CVD tool coating equipment. CVD ceramic hard facing of cutting tools involves the reaction of metal halide vapor with a second gas to form a compound coating (e.g. oxide, nitride, etc.) on the tool. Consequently, the coating retort exhaust often contains solid reaction products which can be collected in a trap located just outside the retort. U.S. Pat. No. 5,261,963 describes condensing excess metal halide gas from CVD aluminizing inside the coating retort using a gettering screen. As is the case with heating the exhaust system, use of internal and/or external exhaust traps or condensers increases both the capital and the maintenance costs of CVD apparatus.
There thus is a need to control and reduce deposition of liquid and/or solid metal from excess gaseous reactant in an exhaust stream of CVD apparatus and to reduce the need for cleaning/repair of the exhaust system between coating runs.
It is an object of the present invention to satisfy these needs.